1300 CLASS MAMMALIA. 



surrounded by a coat of a harder kind of the same substance. The 

 fore-limbs are enormously elongated ; and both the manus and pes 

 are furnished with long, curved claws ; the number of digits never 

 exceeding three in each foot. The skull (fig. 1170) is short, with a 

 descending maxillary process to the zygoma ; and the tail is rudi- 

 mentary. In Bradypus (fig. n 70) the first tooth is equal in size to 

 the second, and is worn horizontally ; while the digits are reduced 

 to two. In Choloepus, however, the first tooth is considerably larger 

 than the second, from which it is separated by a much longer 

 interval than in Bradypus, and wears obliquely ; while there are 

 three digits to each foot. The only known fossil form is from the 



Fig. 1 1 70. — Skull of Bradypus gularis. Recent. South America. Reduced. 



Pleistocene of Argentina, and has been named Nothropus priscus ; 

 it appears to have been about twice the bulk of Bradypus didactylus, 

 and has the first lower tooth separated by a very long interval from 

 the second, although it is of smaller size ; the cranium, upper 

 dentition, and feet are unknown. 



Order IV. Cetacea. — The Cetacea form, perhaps, the most 

 readily defined and sharply differentiated order in the whole class. 

 Their contour is fish-like, the body being fusiform, and passing im- 

 perceptibly into the head without any distinct external neck, and 

 posteriorly gradually tapering to the extremity of the tail, which is 

 furnished with a pair of horizontally-expanded "flukes," formed of 

 dense fibrous tissue covered with skin (fig. n 78). The head is 

 frequently very large, and may be as much as one-third the total 

 length of the animal. The pectoral limbs are reduced to ovoid, 

 paddle-like, organs; and there are no external traces of pelvic limbs. 

 The skin is smooth and without hair ; although bristles may be 

 present in the neighbourhood of the mouth, more especially in 

 young individuals. Frequently there is a median dorsal fin (fig. 

 1 1 78), which however has no bony supports. Both the eye and the 

 external auditory aperture are small ; the former having no nictitating 

 membrane, and the latter no pinna. The nostrils open by a single 



